A fuel assembly of the above kind is disclosed in U.S. Pat. No. 3,697,375. In this and other known fuel assemblies there is a tendency to creep deformations of the walls of the fuel channel. This phenomenon is caused, among other things, by the fuel channel being subjected to fast neutrons in combination with an internal overpressure, resulting in the walls being pressed outwards and acquiring a convex outer side. Creep deformations may thus result in a reduced gap width at some place in the control rod gaps, which may jeopardize the operability of the control rods.
It is possible to counteract, at least to a certain extent, the above-mentioned pressing out of the walls of the fuel channel by providing the fuel channel with a centrally arranged stiffening device, which has a substantially cruciform cross-section along a predominant part of the vertical length of the fuel rods and is constructed in such a way that the fuel channel is divided into four vertical partial channels, each partial channel surrounding one partial bundle which comprises one-fourth of the total number of fuel rods in the fuel assemblies. Such a division into four is described in U.S. Pat. No. 3,164,530, where the stiffening device consists of four perforated dividing plates. Despite the perforation, it cannot be avoided that these dividing plates involve a considerable neutron absorption, which results in a reduced fuel economy.